Optical intrusion alarm system

ABSTRACT

An optical intrusion alarm system employs two photoconductive detectors with regulating feedback to continually adapt the system to changing ambient light. One detector is used to produce an alarm in response to small changes in light caused by movement of an intruder, and the other detector is used to inhibit the first from producing an alarm in response to ambient light transients.

United States Patent 1 Anthony et al.

[111 3,745,550 [4 1 July 10, 1973 OPTICAL INTRUSION ALARM SYSTEM [75] lnventors: Michael P. Anthony, Pasadena;

Arnold J. Goldman, Encino; Stephen L. Kurtin, Pasadena; Charles E. Mullett, Playa Del Rey, all of Califl 3,309,689 3/1967 Keeney 340/258 B Primary Examiner-.Iohn W. Caldwell Assistant Examiner-Glen R. Swann, lll Attorney-Lindenberg, Freilich & Wasserman [73] Assignee: PHENOS, Los Angelos, Calif. [22] Filed: Aug. 2, 1971 [57] ABSTRACT [21] Appl. No.: 168,075 v An optical intrusion alarm system employs two photoconductive detectors with regulating feedback to con- [52] US. Cl. 340/258 B, 250/221 tinuany adapt the system to changing ambient light [51] Int. Cl. G08b 13/18 one detector is used to produce an alarm in response [58] FIG! 0 Search B, 258 D, to Small changes in caused y movement of an 340/258 228 250/221 truder, and the other detector is used to inhibit the first [5.6] References Cited ii 'glrgiglrlogucmg an alarm in response to amhient light UNITED STATES PATENTS 3,564,493 2/1971 l-licklin; 340/258 R 11 Claims, 3.Drawing Figures l4 \7 l O 12 r 2 f R ALARM ALA ALARM RM THRESHOLD ALARM DETECTOR P E DETECTOR DELAV e, L ATCH 1 as 2 2 i, 5- ALARM A ER DETECTOR BUZTZER I GATE LA CH 9 2o 7 i5 5 f F WEE SS SED ALARM ER BUFFER DETECTOR BUZ ER TU RN ON DELAY Pmmnm I 3.145.550

SHEEI 1 [If 2 14 f \7 y 0 12 f ALARM ALARM ALARM THRESHOLD ALARM DETECTOR PREAMP DETECTOR DELAY s, LATCH 22 f L.L ER fi kfigw- BUZZER DETECT R 6 AT E LATCH \9 2O 13 L p r L LLCRER THRESHOLD TUCKER BUFFER DETECTOR T PEEAM P AMP ONE- SHOT TURN-ON DELAY TMJ F 1 1 1 1 j T 1 ER K 1 DETECTOR 4 I DE ECTOR I MICHAEL R ANTHONY j 2 ARNOLD J. GOLD/MAN STEPHEN 1.. KURT/N cmmzs 5. MULLETT A FOP/V5 Y5 l OPTICAL INTRUSION ALARM SYSTEM BACKGROUND OF THE INVENTION This invention relates to an optical system for detecting an intruder into an area, and more particularly to a monitoring system for sounding an alarm upon detecting movement of an intruder in an area subject to a wide range of illumination levels.

Photoconductive devices are ideal and economical for intrusion alarm systems. A given area, such as a room or hall, can be monitored by one or more units, each consisting of a photoconductive device and .acircuit for initiating an alarm upon sensing small changes in lighting caused by movement of an intruder. However, a typical device, such as a cadmium sulphide cell or a photo-transistor, is not useful in an area subject to a wide range of illimination levels, such as a room from early morning to almost total darkness .because such devices change their resistance to .a bias current in proportion to the amount of light striking a sensitive surface. This means that to detect small changes in light, the biasing of a current detection circuit must be continually adjusted for the change in ambient light level.

Another problemwith photo-conductive devices is that their characteristics are quite temperature dependent. This implies that either some means of temperature compensation must-be employed in the detection circuit, or the threshold of detection must be chosen to allow an extra margin for ambient temperature variations. The former adds to the complexity of the detection circuit, and the latter prevents full use of the inherent sensitivity of the system. Moreover, because of the variation in conductance from device to device, replacement of the device would require readjustment of the biasing circuit. Otherwise identical replacement devices must be selected, or extra designmargins must be used in the circuit to allow for device variations. This last alternative would degrade the inherent sensitivity of the system.

Another problem inherent in the use of a photoconductive device in an intrusion alarm is the risk of false alarms due to ambient light flicker, such as commonly occurs with variations in electrical lighting situations due to line voltage variations. Still another'problem is allowing the user to activate the system in the area and then leave, and allowing the user to re-enter the area to deactivate the system without actuating the alarm.

SUMMARY OF THE INVENTION The intrusion alarm system of the present invention provides two light detectors, each comprising a photoconductive device and feedback from a separate preamplifier to regulate the detector bias and thereby compensate for variations in the characteristics of the device and for gradual ambient light level fluctuations. The function of one detector is to initiate an alarm in response to movement of an intruder. That is provided by means for restricting short-term feedback, thereby producing large transient signals in response to abrupt changes in light conditions due to movement of an intruder, and means for limiting the high frequency response to the preamplifier, thereby avoiding a false alarm condition in the system in response to ambient light pulsations as found in fluorescent lamps. The function of the second detector is to inhibit in apositive manner any alarm signal which may be produced by the preamplifier of the alarm detector in the event of ambient light flicker. That function is provided by means for restricting short-term feedback of the second detector as in the alarm detector, and means for limiting the high frequency response of the preamplifier associated with the second detector at a higher frequency than the limitationon the preamplifier associated with the alarm detector. The latter feature simply assures that the flicker detector can .respondmore quickly than the alarm detector. The output of preamplifier associated with the second detector is used to inhibit the output from the preamplifier associated with the alarm detector, thereby further providing immunity from false alarms due to light flicker without degrading the sensitivity of the system to changes in light conditions due to movement of an intruder. Threshold detecting means at the output of the alarm preamplifier detects uninhibited signals from the alarm preamplifier and produces an alarm signal which can be used to activate an alarm device, such as a buzzer.

As a further feature of the invention, the output of the preamplifier associated with the second detector actuates a timing means the output of which further inhibits the output of the alarm preamplifier for a period sufficient to allow the alarm preamplifier to settle. After that period, if the flicker condition is still present, as when a light present in the area being monitored fails, the output of the second preamplifier will continue to inhibit the alarm while the bias in each detec tor is being adjusted for the new level of ambient light.

Further features of the invention include: means for delaying the ability of the system to respond to an alarm condition for a predetermined period after the system has been turned on in order to allow the user time to leave the area being monitored without actuating the alarm; and means for delaying an alarm signal for a predetermined period after an uninhibited signal from the alarm preamplifier has been threshold detected to allow the user time to enter the area and turn the alarm system off without sounding an alarm.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a functional block diagram of the present invention.

FIG. 2 illustrates the preferred orientation of fields of view for two light detectors provided in the system of FIG. 1.

FIG. 3 is a circuit diagram of a preferred embodiment of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to a functional block diagram of a preferred embodiment shown in FIG. 1, two light detectors L0 and 11 are provided, the first to function as an alarm detector in response to small changes in room lighting conditions caused by movement of an intruder, and the second as a detector of flicker, or transient changes, in

the room lighting caused by headlights of a passing car, for example, as well as power line transients caused by the operation of furnaces and other appliances. Each detector comprises a photoconductive device suitably biased. to produce an output signal at a predetermined level until some change in the room lighting occurs. Preamplifiers 12 and 13 amplify the change in the output signals of the respective detectors and 11, and transmit feedback signals over lines 14 and to adjust the bias in each of the detectors 10 and 11 in such a manner as to restore the output signals of the detectors to the predetermined level. This feedback adapts each of the detectors for useful operation over a wide range of illumination levels, such as from dawn to dusk.

Since the function of detector 10 is to initiate an alarm in response to movement of an intruder, it is provided by means for restricting short term feedback. In that manner, abrupt changes in light conditions due to movement of an intruder will produce large transient signals at the output of the alarm preamplifier 12. In order to avoid a false alarm condition in the system in response to ambient light pulsations, such as the pulsing characteristic of a fluorescent light, the alarm preamplifier 12 is provided with means for limiting its high frequency response. Thus, only small changes in room lighting caused by the movement of an intruder will normally produce an alarm signal at the output of the preamplifier 12. Slow or very fast changes will not produce alarm signals because the feedback over the line 14 compensates the alarm detector for slow changes and the high frequency response limitations of the alarm preamplifier causes the very fast changes to be ignored.

It is possible for an ambient light flicker to occur within the frequency response of the alarm preamplifier. The function of the second detector 11 is to inhibit in a positive manner any signal produced by the alarm preamplifier in the event of an ambient light flicker. That is accomplished by limiting the high frequency response of the flicker preamplifier 13 at a higher frequency then the high frequency to which the alarm preamplifier is limited.

In the event of an ambient light flicker, a signal produced by the flicker detector 11 and preamplifier 13 is amplified by a buffer amplifier 16. The amplified transient inhibits any output signal of the alarm preamplifier from being transmitted to an alarn threshold detector and latch circuit 17 by an alarm inhibit gate 18, thereby inhibiting any alarm signal due to a light flicker.

The output of the buffer amplifier 16 also actuates a threshold detector and one-shot circuit 19. The oneshot portion of this circuit causes an alternative inhibit signal which operates through the alarm inhibit gate 18 for a short period greater than any flicker, whatever the cause may be, but less than the duration of small changes in room lighting caused by movement of an intruder. This inhibit period may be typically 4 seconds, a period which is sufficient to allow the alarm preamplifier to settle. in that manner the output of the buffer amplifier 16 will initiate the alarm inhibit function and extend the alarm inhibit function for a predetermined period through the flicker threshold detector and oneshot circuit 19.

If a gross flicker occurs with a duration more than the predetermined period set by the one-shot portion of the circuit 19, such as when a light which provides any of the illumination into the room is turned off for whatever reason, the output of the buffer amplifier 16 will continue to inhibit the alarm signal until the feedback to the detectors via lines 14 and 15 have adjusted both detectors for the new level of illumination. Thus a step change in the illumination level will not produce an alarm, nor will a very brief flicker of a duration less than the period set by the circuit 19. Only changes between those two extremes will cause the alarm threshold detector and latch circuit 17 to be actuated.

Once the alarm threshold detector and latch circuit 17 has been actuated, an alarm buzzer 20 is turned on via an alarm delay circuit 21 and a buzzer latch circuit 22. An alarm delay of typically 10 or 20 seconds is provided to allow the user to re-enter the room and turn the system off without actuating the alarm of the buzzer. The system is turned off by removing the power supply to the various circuits through a master switch S shown in FIG. 3. Once an alarm signal has been transmitted through the alarm delay 21 to the buzzer latch 22, the latter applies power to the alarm buzzer 20 and latches in the buzzer-on condition so that any failure in the alarm threshold detector and latch circuit 17 will not cause power to the alarm buzzer to be turned off.

A turn on delay circuit 23 is connected to the threshold detector and one-shot circuit 19 to actuate that circuit and hold it in its actuated condition beyond its normal timing period of four seconds for a longer period of typically 15 or 30 seconds. This turn-on delay circuit thus overrides the flicker threshold detector and oneshot circuit to cause any alarm signal to be inhibited via the gate 18 for the longer predetermined period after.

power is initially turned on. This is to allow the user to leave the room without actuating the alarm buzzer after turning the system on.

Referring to FIG. 2, the entire system described with reference to FIG. 1 is contained in an enclosure 25 having two apertures 26 and 27. The aperture 26 provides a wider field of view in a generally horizontal direction for the alarm detector and the aperture 27 provides a narrower field of view in a generally vertical direction for the flicker detector.

In practice, the enclosure containing the system is placed in a strategic position, such as a corner of a room with the field of view for the alarm detector generally encompassing the anticipated places of intrusion, namely the doors and windows into the room. Any intrusion will then cause a decrease in light into the room through the door or window being used. However, that is not the only way intrusion can be detected. Any movement in the room which causes a decrease in light received by the alarm detector will cause the alarm buzzer to be actuated.

It should be noted that while reference is made to a room, it should be understood that the area to be monitored need not be a room as such, but may be a hall, vestibule, or the like, and may even be some other area, such as a garage, or an area only partially enclosed, such as a porch, although the area to be monitored will usually be enclosed. I

A preferred embodiment will now be described with reference to a circuit diagram shown in FIG. 3. The alarm detector comprises a photoconductive device 30, such as a cadmium sulfide cell coupled to a variable current source comprising transistors Q and O by a resistor 31. The output of the alarm detector is taken from the collector of the transistor 0, and connected directly to the alarm preamplifier comprising transistors Q and 0,.

Together the transistors 0 and Q4 comprise anoninverting voltage amplifier with a DC gain of approximately 70. The amplifier output appearing at the collector of the transistor 0., is fed back to the alarm detector via the line 14 coupled to the base of the transistor (connected as an emitter follower), by a filter network consisting of resistors 33 to 36 and capacitors 37 and 38. The filter network limits the short term feedback.

The voltage at the collector of the transistor 04 will normally be about +1.5 volts, with a 6-volt power supply, due to the voltage drops across emitter-base func-.

tions of transistors Q and Q and voltage drops across the resistors 34 and 36.

Minimum current in the transistor 0;, is determined by the choice of a resistor 39, and the transistor Q must produce a drop of 0.6V across the resistor 39 in order to turn on the transistor 0;. When the transistor Q, is turned on, it will produce equilibrium in the preamplifier, i.e. it will so adjust the bias current of the alarm detector as to restore the output voltage at the junction between the resistor 31 and the transistor 0, to a level which will produce an output signal from the transistor Q of about 1.5 volts. With the resistor 39 selected to be 22k ohms, this minimum current is approximately 27 ;1.A and results in a voltage at the emitter of the transistor 0 of about 12 mv. The voltage at the base of the transistor 0:, is then about 0.6 volts, which determines the voltage across the photoconductive device 30 and its current limiting resistor 31.

The closed-loop configuration of the preamplifier and the alarm detector will, in this manner, maintain the voltage across the photocond uctive device 30 and current limiting resistor 31 independent of gradual variations in the resistance of the photoconductive device, i.e. independent of gradual variations in illumination. The circuit can accommodate a range of resistances for the photoconductive device of 0.5k ohms to greater than l M ohms. in that manner the feedback from the preamplifier to the alarm detector will cause the alarm detector to adapt itself to an extremely wide range of ambient light conditions.

Abrupt changes in light will produce large transient outputs from the preamplifier even though feedback tends to maintain the output of the alarm detector constant because, as noted hereinbefore', short term feedback is restricted by the capacitors 37 and 38. On the other hand, very high frequency transients, such as may be produced by the flicker of a fluorescent light, will not produce a significant output from the alarm preamplifier because a degenerative feeback capacitor 40 is selected to limit the high frequency response of the preamplifier.

The output of the alarm preamplifier is coupled by a capacitor C to the input of the alarm threshold detector and latch circuit comprised of transistors 0 and Q The transistor 0., is normally off. If a signal coupled by the capacitor C, is of sufficient magnitude and duration, transistor 0,, and Q, will saturate, and remain saturated, because of regenerative feedback through a resistor 41 from the collector of the transistor Q to the base of the transistor 0 The output of this circuit appearing at the collector of the transistor 0,, will be approximately the power supply voltage (6 volts) after the circuit latches.

Normally the transistor 0,, operates as a class A amplifier with its operating point determined by bias resistors 42 and 43. The collector of the transistor 0,, is normally at about 2 volts, and the transistor O is then cut off because its emitter-base junction is back biased by a voltage divider comprised of resistors 44 and 45 which set the threshold level of this alarm detector and latch circuit.

It should be noted that the system shown in F l0. 3 is arranged for detection of a decrease in room lighting which will cause the effective resistance of the photoconductive device 30 to increase, thus causing a decrease in the voltage applied to the base of the transistor Q Because the alarm preamplifier is a noninverting amplifier, this decrease in voltage is coupled as a negative pulse by the capacitor C to the alarm threshold detector and latch circuit. The resulting pulse at the collector of the transistor 0,, will cause the transistor O to conduct and a negative transition will appear at the collector of the transistor Q That negative transition is coupled by the regenerative feedback resistor 41. The result is irrevocable saturation of both transistors Q and 0 v A feedback capacitor 46 from the collector to the base of the transistor 0,, limits the high frequency response of the alarm threshold detector and latch circuit, again to reduce the response of the system to light flickers. A capacitor 47 bypassing the load resistor of the transistor Q causes a delay in the lock-up action of the circuit for a period of approximately 0.2 seconds to insure that the flicker detector channel has had adequate time to detect a flicker and inhibit through the alarm inhibit gate 18, comprised of diodes D and D the signal being coupled by its capacitor C Thus, if the flicker detector channel actuates within 0.2 seconds after a transient from the alarm detector, the alarm threshold detector will not latch up.

After the alarm threshold detector and latch circuit has latched up, an RC-timing circuit (resistor 50 and capacitor 51) will delay the alarm signal from the collector of the transistor Q, for a predetermined period typically 10 to 20 seconds, to allow the user to re-enter the room and deactivate the system by opening the power control switch S, without actuating the alarm. After the transistor Q latches on, the emitter of a transistor Q, in the alarm delay portion of the system is held voltage rises to approximately +4.6 volts, the transistor 0, conducts to actuate thebuzzer latch circuit. A diode D, simply provides a low impedance discharge path for the capacitor 51 to reset the alarm delay quickly when the power switch S is opened. r

The buzzer latch circuit is simply a power amplifier comprised of transistors 08 to Qlo to provide over 300 milliamps to the alarm buzzer 20. The circuit includes regenerative feedback from the collector of the transistor O to the base of the transistor O to drive the tram sistor Q quickly into conduction. In that manner the output transistor Q saturates quickly to avoid over dissipation. Prior to the expiration of the alarm delay, the transistors Q8: Q and Q10 are off. At the end of the delay, the transistor Q conducts and provides base current for the transistor Q The transistor Q then conducts and turns on the transistors Q and 0 The regenerative feedback through the resistor 52 latches the transistors 0 and 0 on so as to latch the buzzer 20 on. A diode D protects the transistors Q from inductive transients caused by the buzzer 20. A shunt capacitor 53 prevents spurious noise from triggering the transistor Q on; otherwise, spurious noise may latch the buzzer on.

Referring now to the flicker detector channel, it should be noted that the flicker detector, preamplifier, and degenerative feedback is the same as forthe alarm detector and preamplifier, except for a capacitor which limits the high frequency response of the flicker preamplifier, corresponding to the capacitor 40 in the alarm preamplifier, which is one-half the value of that capacitor 40 to allow the flicker preamplifier to have greater output from fast transients, i.e. to decrease the limit to high frequency response of the flicker detector channel. Accordingly, the flicker detector 10, preamplifier 13 and degenerative feedback 15 are again illustrated in block diagram form.

The buffer amplifier, comprised of transistor Qrs, is similar to the input stage (transistor Q of the alarm threshold detector and latch circuit A. Any abrupt decrease in light will produce a positive transient at the collector of the transistor Q and if this transient rises high enough to cause a diode D to conduct, the flicker threshold detector and one-shot circuit will be actuated. The threshold level of this circuit is set by resistors 60 and 61 at the same level as the alarm threshold set by resistors 44 and 45, typically at +3 volts with a 6-volt power supply. Accordingly, if a flicker transient is sufficient to trigger the alarm threshold detector and latch circuit, it will also trigger the threshold trigger and one-shot circuit.

However, the latter will be triggered first because of the faster response of the flicker preamplifier. The output of the flicker threshold detector and one-shot circuit taken from the collector of transistor Q will forward bias the diode D to shunt the input to thealarm threshold detector and latch circuit. In the event of a very large transient which may tend to actuate the alarm threshold detector and latch circuit before the flicker threshold detector and one-shot circuit has been actuated to forward bias the diode D the transient will saturate the transistor 0 to forward bias the diode D and thereby inhibit actuation of the alarm threshold detector and latch circuit directly. Diode D, also serves to inhibit actuation of the alarm threshold detector if the transient has a duration longer than that of the oneshot.

The flicker threshold detector and one-shot circuit comprised of transistors Q Q and Q operates as follows: normally the transistor Q is off, andthe transistor Q is saturated. The transistor Q is simply an emitter follower to provide a low impedance to recharge a capacitor 63 quickly. A positive pulse coupled from the transistor Q through the diode D to the base of the transistor 0,, causes a positive pulse to appear at the emitter of the transistor Q11. This pulse is coupled to the base of the transistor Q; via the capacitor 63 and a resistor 64. Conduction of the transistor Q18 is thus interrupted, and its collector swings negative.

1 This negative transition is coupled to the base of the transistor Q through a resistor 65 and the collector of the transistor Q swings positive, thus pulling the base of the transistor Q up. This regenerative feedback latches the one-shot into its temporary state with the transistor Q saturated and the transistor Q18 off. The positive side of the capacitor 63 then discharges toward the ground through a resistor 66, but as it drops to approximately 0.6V below the battery voltage, the transistor Q again conducts. This produces a positive swing at the collector of the transistor 018 which turns off the transistor Q and the circuit returns to its original state. A capacitor 67 does not have a significant effect on the operation of the one-shot operation, but is provided to shunt noise and prevent false triggering.

When the flicker threshold detector and one shot is in its temporary state, the positive voltage of the collector Q16 is coupled through the diode D to the alarm detector channel to prevent triggering of the alarm threshold detector and latch circuit as described hereinbefore. The one-shot circuit is also used by the turnon delay circuit to prevent the alarm threshold detector and latch circuit from being actuated when the user turns on the system by closing the switch S and leaves the room.

The turn-on delay circuit is comprised of an RC- timing circuit including two series connected resistors 71, 72 and a capacitor 73. The junction of the resistors is coupled to the base of the transistor Q by a diode D The positive side of the capacitor 73 is initially at ground, and after the switch S is closed, the capacitor 73 charges toward +6 volts. The majority of the charging current flows from the base of the transistor Q through the diode D and the resistor 72 until the junction between the resistors 71 and 72 is sufficiently positive to back bias the diode D at which time the transistor Q is turned off, and the intrusion alarm system is ready to operate. Until then, while the transistor Q is conducting base current to charge the capacitor 73, the diode D is forward biased to inhibit operation of the alarm threshold and latch circuit. In that manner, the turn on delay will allow sufficient time (determined by the size of the capacitor 73) for the user to leave the room after closing the switch S The diode D in the turn-on delay circuit simply provides a low impedance discharge path for the capacitor 73 when the switch. S is open so that the turn-0n delay circuit may reset quickly when power is shut off.

Once the intrusion alarm system is put into operation, and the period of the turn-on delay has expired, the triggering of the alarm threshold detector and latch circuit can be prevented by saturation of either the transistor 0;; Or the transistor Q in which case the diode D or the diode D will shunt negative transitions coupled by the capacitor C i.e. will couple a positive voltage to the junction of the capacitor C, and resistors.

42 and 43 to inhibit operation of the transistor 0 Although the present invention has been described with reference to a circuit diagram of a preferred embodiment, it should be understood that additional em bodiments and modifications will be obvious to those skilled in the art. Accordingly, it is intended that the claims be interpreted to cover such embodiments and modifications.

What is claimedis:

1. Apparatus for monitoring an area and initiating an alarm signal which can be used to activate an alarm device when light in that area changes in response to movement of an intruder, said apparatus being comprised of first adaptive means for detecting light in said area and producing a first output signal at a predetermined level, said means producing large transients in said first output signal in response to any abrupt change in light received above a predetermined rate of change, means for maintaining said first output signal substantially constant for any ambient light fluctuation below said predetermined rate, and means for limiting the amplitude of said first output signal to below a predetermined level in response to any light pulsation received at a rate of change that is above said predetermined rate by a given amount sufficient not to limit transients in said first output signal caused by abrupt changes in light received due to movement of an intruder,

means for initiating said alarm signal in response to said output signal from said first detecting means, when the amplitude of said first output signal exceeds said pre-determined level,

second adaptive means for detecting light in said area and producing a second output signal at a predetermined level, said means producing large transients in said second output signal in response to any abrupt change in light above a predetermined rate of change, said transients being substantially equal in amplitude to transients produced in said first output signal in response to the same change in light, means for maintaining said second output signal substantially constant for any ambient light fluctuation at rate below said predetermined rate, and means for limiting the amplitude of said second output signal to below said predetermined level in response to any light pulsation received at a rate of change that is above said predetermined rate by an amount sufficiently large to permit the amplitude of said second output signal to be greater than said first output signal for any change in light received at a rate greater than that produced by movement of an intruder, and

means responsive to said second output signal for inhibiting said alarm initiating means when said second output signal exceeds said first output signal in amplitude.

2'. Apparatus for monitoring an area and initiating an alarm signal when light in that area changes in response to movement of an intruder, said apparatus being comprised of first means for detecting light and producing an output signal in response to incident light, said first means including means for limiting changes in said output signal to a predetermined range of rates of change in incident light, said range being selected to include only changes in lighting due to move- I ment of an intruder,

second means for detecting light and producing an output signal in response to incident light, said second means including means for limiting changes in said output signal in said predetermined range of rates of change in incident light to be substantially the same as for said first light detecting means, and means for allowing said second light detecting means to have a greater output signal than said first light detecting means in response to light changes at rates above said range,

alarm threshold detecting means connected to receive at an input terminal the output signal of said first light detecting means for detecting a change above a predetermined threshold,

means responsive to said alarm threshold detecting means for initiating an alarm signal when a change above said predetermined threshold is detected in the output signal of said first light detecting means, and

means responsive to said second light detecting means and connected to said alarm threshold detecting means for inhibiting said alarm threshold detecting means from detecting an output signal of I said first light detecting means above said predetermined threshold when said second light detecting means provides an output signal greater than said output signal of said first light detecting means in response to a change in light above said range.

3. Apparatus as defined in claim 2 including a switch for applying power thereto for operation and wherein said means for initiating an alarm signal includes means for delaying initiation of said alarm signal for a predetermined period to allow a user to turn off the apparatus, by removing power through said switch after entering said area.

4. Apparatus as defined in claim 2 wherein said inhibiting means comprises a first conductive means connected to said input terminal of said threshold alarm detecting means for inhibiting the output signal of said first light detecting means when said second light detecting means provides an output signal greater than said output signal of said first light detecting means in response to a change in light above said range.

5. Apparatus as defined in claim 4 wherein said conductive means comprises a diode, means for back biasing said diode, and means for amplifying and applying said output signal of said second light detecting means to said biasing means for forward biasing said diode.

6. Apparatus as defined in claim 4 including flicker threshold detecting means connected to receive the output signal of said second light detecting means for detecting a change above a predetermined threshold and in response thereto for inhibiting said. alarm threshold detecting means for a predetermined period to allow said first means to settle before resuming normal operation.

7. Apparatus as defined in claim 6 including a switch for applying power thereto for operation and means for inhibiting said alarm threshold detecting means fora predetermined period after said switch applies power to allow a user to turn on the apparatus and, then leave said area.

8. Apparatus as defined in claim 7 wherein said means for initiating an alarm signal includes means for delaying initiation of said alarm signal for a predetermined period to allow a user to turn off the apparatus by removing power through said switch after entering said area.

9. Apparatus as defined in claim 6 wherein said flicker threshold detecting means comprises threshold means for detecting when said second light detecting means provides an output signal which exceeds a predetermined level, timing means for producing an inhibit signal for said predetermined period after said threshold means detects that. said level has been exceeded, and a second conductive means connected to said, input terminal of said alarm threshold detecting means for inhibiting the output signal of said first light detecting means for said predetermined period of said timing means.

10. Apparatus as defined in claim 9 including a switch for applying power thereto for operation and means for actuating said timing means when said switch applies power, thereby inhibiting said alarm threshold detecting means for said predetermined period to allow a user to turn on the apparatus and then leave said area.

in light above said range, and said second conductive means comprises a diode, means for back biasing said second diode, and means for applying said inhibit signal from said flicker threshold detecting means to said means for back biasing said second diode for forward biasing said second diode.

* t a s 

1. Apparatus for monitoring an area and initiating an alarm signal which can be used to activate an alarm device when light in that area changes in response to movement of an intruder, said apparatus being comprised of first adaptive means for detecting light in said area and producing a first output signal at a predetermined level, said means producing large transients in said first output signal in response to any abrupt change in light received above a predetermined rate of change, means for maintaining said first output signal substantially constant for any ambient light fluctuation below said predetermined rate, and means for limiting the amplitude of said first output signal to below a predetermined level in response to any light pulsation received at a rate of change that is above said predetermined rate by a given amount sufficient not to limit transients in said first output signal caused by abrupt changes in light received due to movement of an intruder, means for initiating said alarm signal in response to said output signal from said first detecting means, when the amplitude of said first output signal exceeds said predetermined level, second adaptive means for detecting light in said area and producing a second output signal at a predetermined level, said means producing large transients in said second output signal in response to any abrupt change in light above a predetermined rate of change, said transients being substantially equal in amplitude to transients produced in said first output signal in response to the same change in light, means for maintaining said second output signal substantially constant for any ambient light fluctuation at rate below said predetermined rate, and means for limiting the amplitude of said second output signal to below said predetermined level in response to any light pulsation received at a rate of change that is above said predetermined rate by an amount sufficiently large to permit the amplitude of said second output signal to be greater than said first output signal for any change in light received at a rate greater than that produced by movement of an intruder, and means responsive to said second output signal for inhibiting said alarm initiating means when said second output signal exceeds said first output signal in amplitude.
 2. Apparatus for monitoring an area and initiating an alarm signal when light in that area changes in response to movement of an intruder, said apparatus being comprised of first means for detecting light and producing an output signal in resPonse to incident light, said first means including means for limiting changes in said output signal to a predetermined range of rates of change in incident light, said range being selected to include only changes in lighting due to movement of an intruder, second means for detecting light and producing an output signal in response to incident light, said second means including means for limiting changes in said output signal in said predetermined range of rates of change in incident light to be substantially the same as for said first light detecting means, and means for allowing said second light detecting means to have a greater output signal than said first light detecting means in response to light changes at rates above said range, alarm threshold detecting means connected to receive at an input terminal the output signal of said first light detecting means for detecting a change above a predetermined threshold, means responsive to said alarm threshold detecting means for initiating an alarm signal when a change above said predetermined threshold is detected in the output signal of said first light detecting means, and means responsive to said second light detecting means and connected to said alarm threshold detecting means for inhibiting said alarm threshold detecting means from detecting an output signal of said first light detecting means above said predetermined threshold when said second light detecting means provides an output signal greater than said output signal of said first light detecting means in response to a change in light above said range.
 3. Apparatus as defined in claim 2 including a switch for applying power thereto for operation and wherein said means for initiating an alarm signal includes means for delaying initiation of said alarm signal for a predetermined period to allow a user to turn off the apparatus, by removing power through said switch after entering said area.
 4. Apparatus as defined in claim 2 wherein said inhibiting means comprises a first conductive means connected to said input terminal of said threshold alarm detecting means for inhibiting the output signal of said first light detecting means when said second light detecting means provides an output signal greater than said output signal of said first light detecting means in response to a change in light above said range.
 5. Apparatus as defined in claim 4 wherein said conductive means comprises a diode, means for back biasing said diode, and means for amplifying and applying said output signal of said second light detecting means to said biasing means for forward biasing said diode.
 6. Apparatus as defined in claim 4 including flicker threshold detecting means connected to receive the output signal of said second light detecting means for detecting a change above a predetermined threshold and in response thereto for inhibiting said alarm threshold detecting means for a predetermined period to allow said first means to settle before resuming normal operation.
 7. Apparatus as defined in claim 6 including a switch for applying power thereto for operation and means for inhibiting said alarm threshold detecting means for a predetermined period after said switch applies power to allow a user to turn on the apparatus and, then leave said area.
 8. Apparatus as defined in claim 7 wherein said means for initiating an alarm signal includes means for delaying initiation of said alarm signal for a predetermined period to allow a user to turn off the apparatus by removing power through said switch after entering said area.
 9. Apparatus as defined in claim 6 wherein said flicker threshold detecting means comprises threshold means for detecting when said second light detecting means provides an output signal which exceeds a predetermined level, timing means for producing an inhibit signal for said predetermined period after said threshold means detects that said level has been exceeded, and a second conductive means connected to said input terminal of saId alarm threshold detecting means for inhibiting the output signal of said first light detecting means for said predetermined period of said timing means.
 10. Apparatus as defined in claim 9 including a switch for applying power thereto for operation and means for actuating said timing means when said switch applies power, thereby inhibiting said alarm threshold detecting means for said predetermined period to allow a user to turn on the apparatus and then leave said area.
 11. Apparatus as defined in claim 7 wherein said first conductive means comprises a diode, means for back biasing said diode, and means for amplifying and applying said output signal of said second light detecting means to said biasing means for forward biasing said first diode when said second light detecting means provides an output signal greater than said output signal of said first light detecting means in response to a change in light above said range, and said second conductive means comprises a diode, means for back biasing said second diode, and means for applying said inhibit signal from said flicker threshold detecting means to said means for back biasing said second diode for forward biasing said second diode. 